Recent estimates suggest that the number of U.S. men with erectile dysfunction may be near 10 to 20 million, and inclusion of individuals with partial erectile dysfunction increases the estimate to about 30 million. Erectile dysfunction has a number of etiologies, including neuropathy and vascular disease. The male erectile response is initiated by the action of neurons, or nerve cells (i.e., neuronal action), and is maintained by a complex interplay between events involving blood vessels (i.e., vascular events) and events involving the nervous system (i.e., neurological events).
The part of the nervous system that regulates involuntary action (e.g., the intestines, heart, glands) is the autonomic nervous system. The autonomic nervous system is divided into two mutually antagonistic, physiologically and anatomically distinct systems: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system originates in the thoracic and lumbar regions of the spinal cord, and in general, opposes the physiological affects of the parasympathetic nervous system. For instance, the sympathetic system tends to reduce digestive secretions or speed up the heart, usually when an individual is in an active state. The parasympathetic nervous system originates in the brain stem and the lower part of the spinal cord, and, in general, opposes the physiological effects of the sympathetic nervous system. Thus, the parasympathetic nervous system tends to stimulate digestive secretions or slow the heart, usually when an individual is in a relaxed state.
It is parasympathetic neuronal action that initiates the male erectile response. Specifically, this parasympathetic input originates from the pelvic splanchnic nerve plexus. The pelvic splanchnic nerve plexus is comprised of branches from the second, third, and fourth sacral nerves (from the lower part of the spinal cord) that intertwine with the inferior hypogastric plexus, which is a network of nerves in the pelvis. The cavernous nerves (designated greater and lesser) are derived from the pelvic splanchnic nerves, via the prostatic plexus, and supply parasympathetic fibers to the corpora cavernosa and corpus spongiosum, the spongy tissues in the penis that are engorged with blood during an erection. The corpora cavernosa are two paired tissue bodies that lie dorsally in the penis, while the corpus spongiosum is located ventrally and surrounds the urethra. The corpus spongiosum expands at the terminal end to form the glans penis. These erectile tissues are composed of venous spaces lined with epithelial cells separated by connective tissue and smooth muscle cells.
Parasympathetic activity allows erection by relaxation of the smooth muscle (i.e., muscle found in the walls of internal organs, blood vessels, hair follicles, etc. that contracts without voluntary control) and dilation of the helicine arteries, which are arteries found in the erectile tissue of the penis. The dilation of the arteries causes greatly increased blood flow through the erectile tissue, which leads to expansion of the corpora cavernosa and the corpus spongiosum. As the corpora cavernosa and the corpus spongiosum expand, the venous structures draining the penis are compressed against the fascia surrounding each of the erectile tissues (i.e., the tunica albuginea of the corpora cavernosa and the tunica albuginea of the corpus spongiosum). Thus, the outflow of blood is restricted, and the internal pressure increases. This vein-obstruction process is referred to as the corporal veno-occlusive mechanism.
Conversely, sympathetic innervation from the hypogastric nerves and/or certain nerves of the inferior hypogastric plexus, which derive from the sympathetic ganglia, inhibit parasympathetic activity and cause constriction of the smooth muscle and helicine arteries, making the penis flaccid. The flaccid state is maintained by continuous sympathetic (alpha-adrenergic) nervous system stimulation of the penile blood vessels and smooth muscle.
Erectile dysfunction has a number of causes, both physiological and psychological, and in many patients the disorder may be multifactorial. The causes include several that are essentially neurologic in origin. Damage to the spinal cord may produce varying degrees of erectile failure depending on the location and severity of the damage. Damage to the pathways used by the autonomic nervous system to innervate the penis may interrupt “psychogenic” erection initiated by the central nervous system. Damage to somatic nervous pathways may impair reflexogenic erections and may interrupt tactile sensation needed to maintain psychogenic erections. Not only do traumatic lesions affect erectile ability, but disorders leading to peripheral neuropathy may impair neuronal innervation of the penis or of the sensory afferents. The endocrine system itself, particularly the production of androgens, appears to play a role in regulating sexual interest, and may also play a role in erectile function.
Erectile dysfunction is a common complication of prostate surgery, such as prostatectomy (surgical removal of all or part of the prostate), which is a mainstay of treatment for prostate cancer. Approximately 180,000 new cases of prostate cancer will occur in the US each year, with 35,000 men expected to die of the disease annually. A January 2000 study of 1,042 men diagnosed with primary prostate cancer and who underwent radical prostatectomy for localized prostate cancer showed that at least 18 months following surgery, 59.9 percent were impotent and 8.4 percent were incontinent. At 24 months, 59.9 percent of men reported that erections were not firm enough for sexual intercourse, and 44.2 percent were unable to have any erections.
Among men who were not impotent before surgery, the proportion of men who reported being impotent 18 or more months after surgery varied according to whether a nerve-sparing procedure was attempted. Nerve-sparing procedures attempt to leave intact one or both of the “neurovascular bundles” which pass close to the prostate capsule. In most cases, the “bundles” are essential for achieving and maintaining an erection. In the January 2000 study, 65.6 percent of non-nerve-sparing, 58.6 percent of unilateral nerve-sparing, and 56.0 percent of bilateral nerve-sparing procedures produced impotence. Despite the level of urinary incontinence and sexual dysfunction reported in this study, most men (71.5 percent) reported they would choose radical prostatectomy again.
To achieve improved outcomes in nerve-sparing surgery, devices are available for intra-operative cavernous nerve simulation, often with penile tumescence monitoring. The UroMed CaverMap™ Surgical Aid is an example of such a device. The CaverMap™ is an acute neurostimulator used to stimulate the cavernous nerves during prostate surgery. Upon such stimulation, the penis becomes erect within 20 seconds to 1 minute. During a typical procedure, the CaverMap™ Surgical Aid is used initially to establish the baseline erectile response to stimulation via stimulation bilaterally at the posterolateral urethra. As the surgery progresses and the neurovascular bundle is visualized, the CaverMap™ is used to stimulate bilaterally along the lateral pedicles at the apex, mid, and base of prostate. Part or all of the prostate and seminal vesicles are removed, sparing those portions containing the cavernous nerves.
There are few good options for men suffering from erectile dysfunction following prostatic surgery. A well-publicized oral medication, sildenafil citrate (available from Pfizer Inc. of New York, N.Y.) under the trademarked name Viagra®), is available, but requires an hour to exert its full effects, and may have significant side effects such as abnormal vision, flushing, headache, and diarrhea. Vardenafil is a medication undergoing clinical investigation, which has a mechanism of action similar to sildenafil. Despite its drawbacks, the ability to preserve erectile function following prostate surgery has been favorably affected by the availability of sildenafil. Sildenafil appears to be most effective when there is some remaining erectile function.
Intracavernosal injection therapy, in which a patient injects vasodilator substances (e.g., alprostadil, papaverine, phentolamine) into the corpora of the penis, suffers a high rate of patient dropout. The most commonly used drug is alprostadil. Alprostadil is naturally occurring prostaglandin E1, or PGE1, that is present in the penis and is involved in the natural erection process. (Thus, “alprostadil”, “prostaglandin E1”, and “PGE1” are used interchangeable herein.) Alprostadil has been used in the treatment of impotence in the UK since 1994. Alprostadil relaxes the blood vessels and muscles in the erectile tissue of the penis allowing increased blood flow, the basis of a normal erection.
Intracavernosal injection therapy suffers a high rate of patient dropout, as does the therapeutic application of vacuum constriction devices. Several forms of penile prostheses are available, including semirigid, malleable, and inflatable, but these have significant problems with mechanical failure, infection, and device erosion. As has been shown, various stimulation devices and medications have been proposed for treating erectile dysfunction, most with significant drawbacks.